In recent years, attention is paid to fuel cells characterized by high energy conversion efficiency and not producing hazardous materials as a result of generating electricity. A solid polymer electrolyte fuel cell operating at a temperature of 100° C. or lower is known as one type of such fuel cells.
A solid polymer electrolyte fuel cell is a power generating device that has a basic structure in which a solid polymer electrolyte membrane is sandwiched between a fuel electrode and an air electrode. Hydrogen is supplied to the fuel electrode and oxygen is supplied to the air electrode. A solid polymer electrolyte fuel cell generates electricity according to the following electrochemical reaction.Fuel electrode: H2->2H++2e−  (1)Air electrode: ½O2+2H++2e−->H2O  (2)
Each of the fuel electrode and the air electrode is composed of a laminate of a catalytic layer and a gas diffusion layer. The catalytic layers of the electrodes are opposed to each other, sandwiching the solid polymer electrolyte membrane, thus constituting the fuel cell. The catalytic layer is formed by bonding catalyst-carrying carbon particles on an ion exchange resin. The gas diffusion layer provides a passage for oxygen or hydrogen. Generation of electricity takes place in a triphasic interface between the catalyst in the catalytic layer, the ion exchange resin and a reactant (hydrogen or oxygen).
At the fuel electrode, hydrogen included in the supplied fuel is split into hydrogen ions and electrons according to the equation (1) shown above. Hydrogen ions move toward the air electrode via the interior of the solid polymer electrolyte membrane. Electrons move toward the air electrode via an external circuit. At the air electrode, oxygen included in the oxidant supplied to the air electrode reacts with hydrogen ions and electrons arriving from the fuel electrode, so as to produce water according to the equation (2) shown above. As a result of electrons moving from the fuel electrode to the air electrode, electric power is retrieved.
In the related art, the solid polymer electrolyte membrane is formed of fluorinated polymer material such as perfluoroacid polymers including Nafion (TM) from DuPont, Flemion (TM) from Asahi Glass Company and Aciplex (TM) from Asahi Kasei Corporation.
A problem with fluorinated polymers is that production thereof is difficult and expensive. This has created a hurdle for reduction to practice and commercialization of a fuel cell for home use, automotive use and civilian use. Furthermore, fluorinated polymer materials such as Nafion present a problem in that moisture control is difficult and heat endurance is poor.
In recent years, development of a solid polymer electrolyte fuel cell of a type called a direct methanol fuel cell (hereinafter, referred to as DMFC), in which methanol is directly supplied as fuel to a fuel electrode, is underway. A related-art fluorinated polymer material to form a solid polymer electrolyte is characterized by a relatively poor capability to block methanol. When a fluorinated polymer is used as an electrolyte membrane in a DMFC, crossover of methanol occurs so that a desired cell characteristic cannot be obtained.
Accordingly, there is a need for development of a new solid polymer electrolyte membrane in which the aforementioned problem is eliminated. For example, a patent document 2 discloses use of non-fluorinated polymer of the sulfonic acid type which, does not contain fluorine, as a solid polymer electrolyte material to form a solid polymer electrolyte membrane. A patent document 3 and a patent document 4 describe use of non-fluorine polymer of the sulfonic acid type which does not contain fluorine, as a solid polymer electrolyte material to form an electrolyte membrane having an excellent capability to block alcohol such as methanol.
Patent document 1: Japanese Laid-Open Patent Application No. 2002-63915
Patent document 2: Japanese Laid-Open Patent Application No. 10-45913
Patent document 3: Japanese Laid-Open Patent Application No. 2002-105200
Patent document 4: Japanese Laid-Open Patent Application No. 2002-216802